The electrostatic coating process for the application of color coatings on various substrates is well known. The composition which is utilized in this process is an electrically chargeable powder mixture of a film-forming polymer and a pigment (or pigments) having the desired color. Unfortunately, preparation of such powder coating compositions for application by a powder spray applicator has presented a problem in the past in connection with platy pigments.
Platy pigments of a metallic material rely on their laminar structure for maximum appearance effect. Such plates include, for instance, metallic flakes such as aluminum, bronze and stainless steel plates as well as natural or synthetic pearlescent pigments exemplified by, e.g., natural pearlescence or a metal oxide-coated substrate such as titanium dioxide-coated mica, iron oxide-coated mica, titanium dioxide-coated glass, iron oxide-coated glass and iron-coated aluminum flakes. The laminar structure of such metallic or pearlescent pigments is destroyed during the extrusion or grinding processes which are used in the manufacture of powder coating compositions and as a result, the coating appearance achieved exhibits a reduced luster effect.
The industry has attempted to avoid the foregoing problem by dry blending the pigments and polymer powder, i.e., the polymer carrier is mechanically blended with the pigment. Unfortunately, the pigment and the powder particles usually develop a different charge magnitude which results in a color shift when the powder coating composition is electrostatically sprayed on a substrate. In addition, the pigments tend to separate from the powder in that volume of the powder spray composition which does not attach to the substrate and is then recovered, which makes that recovered material difficult to reuse.
To overcome the problems encountered as a result of the dry blending process, a blending process in which the powder base and the pigment were mixed and then heated to a temperature sufficient to soften the surface of the powder particles so that the pigment could bind to the surface of such particles was developed, as described in U.S. Pat. No. 5,187,220. While this process works well for corona electrostatic charging and alleviates the problems with reusing the powder overspray, it also leaves two distinctly different surfaces in the final mixture. The procedure also does not work well for tribo electrostatic charging.
A significant advance in the art is described in U.S. Pat. No. 5,824,144, the disclosure of which is hereby incorporated by reference. As there described, the metal containing platelet pigment is provided with a viscous surface layer of polymer or other sticky liquid material. When that treated pigment is blended with powder coating composition, the powder attaches to the surface of the pigment thereby minimizing color separation. The pigment is also encapsulated by the powder to thereby present a single surface. A better charge of those pigment particles which do not become attached to the powder materials is also realized.
A significant problem in connection with the use of metal-containing platelet pigments in a powder coating composition is that the concentration of the pigment which can be incorporated is limited. While pigment amounts of up to about 10% or more can be utilized in single application environments or in the laboratory, the concentration is limited to about 3% when operating on a larger scale as is required commercially. At levels higher than about 3%, numerous problems arise. These include spray gun clogging, blotchy panel appearance and color separation.
It has now been discovered that a treatment which had previously been developed for pearlescent pigments intended for exterior use to provide improved humidity, resistance and overall weatherability and which was used in liquid coating systems such as a solvent or water borne automotive paint systems, for example, pigmented base coat and clear top coat, provides superior properties in the context of powder coatings. The exterior pigment treatment surprisingly improves the application properties of the pigment when incorporated into powder coatings. Such improvements include less build up of the pigment at the electrostatic gun tip, improved transfer through feed lines, improved transfer efficiency of the pigment and a more uniform coating appearance.
The present invention relates to an improved pearlescent pigment powder coating composition and powder coating method. More particularly, the invention relates to a powder coating composition comprising a mixture of a particulate resin carrier and a particulate pigment in which the pigment is a laminar metal-containing pigment having a first coating of a hydrated aluminum oxide or hydrated cerium and aluminum oxides and a coating of a hydrolyzed silane coupling agent on or intermingled with the first coating. Use of this powder coating composition provides an improved powder coating application method.
The powder coating composition of the present invention is the conventional powder coating composition for electrostatic application as a result of dry blending in which the pigment has been treated as described below. Thus any known particulate resin carrier used heretofore can also be used in the composition and process of the present invention. As a result of the improved pigment utilized in the present invention, the pigment concentration can be typically up to about 8% of the total powder. Typically, depending on the effect desired, the pigment will constitute about 1 to 8% of the composition.
The pigment of the present invention is laminar, that is, it is a platelet-like pigment. The platelets can be made of a reflective metal such as aluminum, bronze or stainless steel in the form of flakes or also can be any natural or synthetic pearlescent pigment. Natural pearlescent pigments include natural pearlescence and the synthetic pigments include metal oxide-coated mica pigments, metal oxide-coated glass flakes, iron-coated aluminum flakes and reduced titanium-coated micas. On a commercial basis, the titanium dioxide and iron oxide micas are encountered most often. Nevertheless, any laminar pigment can be treated in accordance with the present invention.
Coating of a metal oxide-coated mica pigment with a hydrous aluminum oxide per se is known. It is described, for example, in U.S. Pat. No. 5,091,011, the disclosure of which is incorporated herein by reference. Briefly, the pigment is dispersed by stirring in water and then an aluminum compound such as aluminum chloride, aluminum sulfate or aluminum potassium sulfate, and a neutralizing agent such as sodium hydroxide, potassium hydroxide ammonia or urea, are added simultaneously as aqueous solutions. The resulting hydrolysis causes the hydrous oxide to deposit on the substrate. As described, the aluminum compound must be added slowly enough to permit the formation of a smooth, continuous layer on the platelets and the rate should fall within the range of about 0.03 to 0.1 mg Al per minute per gram of pigment, preferably about 0.005 to 0.07 mg Al/min/g pigment. A quantity of aluminum compound solution is used so as to produce a hydrous aluminum oxide coating containing about 0.05 to 1.2% aluminum, preferably about 0.1 to 0.8% aluminum, based on the total weight of the pigment. Pigments in which the concentration of aluminum is about 1.2% are less effective in stabilization than lower concentrations. After deposition of the coating, the product can be filtered, washed with water and dried at any convenient temperature. Use of a temperature which is high enough to calcine the hydrous aluminum oxide should be avoided.
The coating of the pigments with a coating consisting essentially of a combination of hydrated cerium and aluminum oxides is also per se known. It is described, for instance, in U.S. Pat. No. 5,423,912, the disclosure of which is incorporated herein by reference. Briefly, the pigment is dispersed in a liquid from which the cerium and aluminum can be readily precipitated onto the surface of the pigment. This conveniently, and preferably, is an aqueous dispersion. The solid pigment in the dispersion generally comprises about 5 to 30%, preferably about 10 to 20%, and the cerium and aluminum are each added to the dispersion in the form of a salt which is soluble in a liquid medium. While other salts can be used, the nitrate salts are preferred. It is also preferred to deposit about 0.1-1.5% cerium hydroxide, more preferably 0.2-0.6%, calculated as weight percent cerium and about 0.1-1%, more preferably 0.2-0.6%, aluminum hydroxide, calculated as weight percent aluminum, based on the weight of the pigment. The salts can be added to the slurry individually in either order and precipitated or preferably, added simultaneously and precipitated. Precipitation is controlled by raising the pH to a value greater than about 5, preferably to a value of about 5.5-7.5. After completion of the precipitation step, the treated product is separated from the dispersion by any convenient means such as, for instance, filtration, centrifugation or settling, washed and dried.
The aluminum- or aluminum-cerium-treated pigment of this invention is additionally treated with a hydrolyzed silane coupling agent or a mixture of such agents. These, as known, are compounds which act as an interface between an organic material and an inorganic material to enhance the affinity between the two. Thus, the silane coupling agents generally have both an organo functional group and a silicon functional group bonded either directly or indirectly to silicon. The silicon functional groups are generally alkoxy groups and preferably C1-4 alkoxy groups.
Examples of silane coupling agents which can be used in the present invention are gamma-(2-aminoethyl)aminopropyltrimethoxysilane, aminopropyl trimethoxysilane, gamma-aminopropyltriethoxysilane, gamma-(2-aminoethyl)aminopropylmethyldimethoxysilane, gamma-methacyryloxypropylmethyltrimethoxysilane, gamma-metacyryloxypropyltrimethoxysilane, gamma-glycidoxypropyl trimethoxysilane, gamma-mercaptopropyltrimethoxysilane, vinyltriacetoxysilane, gamma-chloropropyltrimethoxy silane, vinyltrimethoxysilane, trimethylchlorosilane, gamma-isocyanatopropyltriethoxysilane and the like.
The silane coupling agent should be selected so that it is suitable for any organic material which will be combined with the pigment in use.
The pigment is treated with the silane coupling agent by dry or wet mixing.
For instance, an aqueous solution of the agent in water or a mixture of water and an organic solvent can be added to an aqueous slurry of the pigment. The silane is preferably prehydrolyzed such as, for instance, by stirring the coupling agent in water for a suitable period of time. It is also possible to effect hydrolyzation at the time of mixing. In general, about 0.1 to 10 wt %, preferably about 0.25 to 5 wt %, of the silane coupling agent is used based on 100 parts by weight of pigment being treated. The coupling agent and pigment are combined for a period of time sufficient to allow reaction to occur, which may last from a few minutes to several hours or more, preferably about 3 to 24 hours. Thereafter the treated pigment can be recovered in the conventional fashion such as by filtration, centrifugation and the like, and dried. It is also possible to combine, if desired, the coupling agent treatment with the aluminum/cerium treatment.